In general, a direct-current stabilizer is used to supply a DC voltage necessary for electronic devices. For example, a so-called dropper-type direct-current stabilizer which outputs a stabilized voltage by decreasing an input voltage is commonly used because it has low noise and is easy to design. The following description discusses such direct-current stabilizers.
As illustrated in FIG. 19, in a direct-current stabilizer, for example, when an input voltage applied to an input terminal IN reaches a predetermined level, an actuator 81 starts operating and a reference voltage circuit 82 generates a reference voltage. An output voltage V.sub.O delivered to an output terminal OUT is divided by resistors R.sub.81 and R.sub.82. The difference between the resulting voltage and the reference voltage is amplified by a differential amplifier 83.
The differential amplifier 83 controls the base current of a transistor Tr.sub.81 through a transistor Tr.sub.82 by adjusting an output according to the difference. The transistor Tr.sub.81 is an n-p-n transistor for controlling output, and stabilizes the output voltage V.sub.O by controlling the base currents. The output voltage V.sub.O is applied to a load 84. The output characteristics of the output voltage V.sub.O is improved by a capacitor C.sub.81 connected to the output terminal OUT in parallel with the load 84.
When the collector current of the transistor Tr.sub.81 is increased by a short circuit or overload, the voltage drop in a resistor R.sub.83 connected to the emitter of the transistor Tr.sub.81 becomes significant. Then, the base-emitter voltage of a transistor Tr.sub.83 for controlling current is significantly increased. This causes the transistor Tr.sub.83 to be switched on, and the base currents of the transistors Tr.sub.82 and Tr.sub.81 to be limited. Consequently, the collector current of the transistor Tr.sub.81 is limited, and the transistor Tr.sub.81 is protected from an overcurrent.
Another direct-current stabilizer shown in FIG. 20 includes a p-n-p transistor Tr.sub.84 for controlling output. Similar to the above-mentioned n-p-n transistor, with the p-n-p transistor Tr.sub.84, the reference voltage circuit 82 generates a reference voltage with the operation of the actuator 81, an output voltage is divided by the resistors R.sub.81 and R.sub.82, and a difference between the divided voltage and the reference voltage is amplified by the differential amplifier 83. The base current of the transistor Tr.sub.84 is controlled by the output of the differential amplifier 83 through a transistor Tr.sub.85 as driver, and thereby stabilizing the output voltage V.sub.O.
When the collector current of the transistor Tr.sub.84 is increased by a short circuit or overload, the collector current of the transistor Tr.sub.85 is also increased. The base-emitter voltage of a transistor Tr.sub.86 for controlling current is increased by a resistor R.sub.84 which is connected in series with the emitter of the transistor Tr.sub.85. This causes the transistor Tr.sub.86 to be switched on, and the base currents of the transistors Tr.sub.85 and Tr.sub.84 to be limited. As a result, the collector Current of the transistor Tr.sub.84 is limited and the transistor Tr.sub.84 is protected from an overcurrent.
However, since the former direct-current stabilizer uses the n-p-n transistor Tr.sub.81 to control the output voltage, the collector-emitter voltage drops significantly, causing considerable losses and poor efficiency.
On the other hand, the latter direct-current stabilizer uses the p-n-p transistor Tr.sub.84 for controlling the output voltage so as to reduce the losses by minimizing the potential difference between the input voltage and the output voltage. However, such a direct-current stabilizer also has the following problem.
The p-n-p transistor usually can not produce a direct current gain that a n-p-n transistor of the same chip size produces. Therefore, in order to produce a direct current gain similar to the gain of the n-p-n transistor of the same rating, it is necessary to increase the size of the chip, resulting in an increase in costs.
Moreover, the direct-current stabilizer using a p-n-p transistor presents the following structural problem.
A direct-current stabilizer shown in FIG. 21 has a structure where a transistor section 91 as a transistor for controlling an output voltage and an IC section 92 for controlling the transistor are vertically arranged on a single chip. More specifically, such a direct-current stabilizer has a Complicated structure where an n+ buried layer 94 and a p.sup.+ buried layer 95 are formed in this order on a p-type substrate 93. In addition, there is a need to provide a p-well region 96 to form the p-n-p structure. Therefore, the number of wafer processes in manufacturing is increased, resulting in an expensive chip. Furthermore, an increase in the number of heat treatment processes causes the diffused layers 97 through 100 to expand, thereby increasing the area and costs of the chip.
A direct-current stabilizer shown in FIG. 22 has a structure where a transistor section 101 as a transistor for controlling output voltage and an IC section 102 for controlling the transistor are laterally arranged on a single chip. In such a direct-current stabilizer, an emitter diffused layer 104, a base diffused layer 105 and a collector diffused layer 106 are formed in a cross direction on an n-type epitaxial layer 103. This arrangement causes the chip to have an increased area, resulting in an increase in costs. The characters (B), (C) and (E) in the drawing represent the base, collector and emitter, respectively.